This invention relates to in situ recovery of shale oil, and more particularly, to techniques for attentuating airblast produced when detonating large amounts of explosive for forming an in situ oil shale retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil."
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products of retorting. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting." Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooled oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
U.S. Pat. No. 4,043,595, and 4,043,596 disclose methods for explosively expanding formation containing oil shale to form an in situ oil shale retort. According to methods disclosed in those patents, formation is excavated to form a columnar void bounded by unfragmented formation having a vertically extending free face, drilling blast holes adjacent the columnar void and parallel to the free face, loading the blast holes with explosive, and detonating the explosive in a single round. This explosively expands the formation adjacent the columnar void toward the free face for forming a fragmented permeable mass of formation particles containing oil shale in an in situ oil shale retort. In one embodiment, the columnar void is a slot having large, parallel, planar vertical free faces toward which the formation in the retort site can be explosively expanded. The blast holes are arranged in planes parallel to such free faces. An embodiment of such a method is described in my copending U.S. patent application Ser. No. 790,350, filed Apr. 25, 1977, now U.S. Pat. No. 4,118,071 which is incorporated herein by this reference.
Explosive in such blast holes is detonated in a time-delay sequence for explosively expanding unfragmented formation within the retort site in a single round of a plurality of explosions. The sequence of blasting is rapid, and in an embodiment disclosed in U.S. Pat. No. 4,118,071, time-delays for explosively expanding formation toward the columnar void span a time period of less than 200 milliseconds. Shorter time delays can be used in other embodiments. In one embodiment, as much as 85 tons of explosive are detonated in a single round of explosions for explosively expanding formation toward a columnar void. This produces a powerful explosion which generates a large volume of gas within the retort site. At least a portion of the gas must escape from the mine workings, and since high pressures are present in the vicinity of the blast, very high flow velocities can occur in the mine workings. This is known as airblast which can travel through underground workings leading away from the blast site. Airblast from such a powerful explosion can cause serious damage to equipment and injury to personnel in such underground workings. Equipment which can be damaged from such airblast cannot necessarily be easily or economically removed from underground workings prior to such explosive expansion and then returned after blasting. Thus, there is a need to attentuate airblast produced when detonating large amounts of explosive for forming an in situ oil shale retort for reducing effects of such airblast in the mine workings adjacent the blast site.